To date, there has been known an idling stop system that automatically stops an engine in order to reduce the fuel cost of the automobile when a predetermined condition is established. Furthermore, there has been proposed an engine starting apparatus in which even when the ring gear coupled with the crankshaft of the engine is being rotating, the pinion gear coupled with the output axle of the starter motor is engaged with the ring gear so that in the case where immediately after the engine combustion is stopped by the idling stop system, an engine restart demand is issued when the engine is in an inertial rotation, the engine is restarted as quickly as possible.
With regard to these conventional engine starting apparatuses, there has been proposed a technology in which in order to make the rotating ring gear and the pinion gear engage with each other, the rotation speed of the engine in an inertial rotation is detected, the timing is set in such a way that the rotation speed of the ring gear at a time when the pinion gear abuts against the ring gear falls within a predetermined range, and at the timing when the rotation speed of the ring gear falls within the predetermined range, the pinion gear is engaged with the ring gear so that the engine is restarted. Moreover, when an engine rotates backward, the engagement between the pinion gear and the ring gear causes large impact, and hence excessive engagement noise and the abrasion in the pinion gear and the ring gear are caused; thus, there has been proposed a technology in which under a predetermined condition, the operation of making the pinion gear engage with the ring gear is prohibited.
That is to say, International Publication No. WO 2014-054471 (Patent Document 1) discloses an engine starting apparatus that controls restarting of an engine, based on the output of a rotation sensor that detects the rotation speed of the engine, while regarding the section where the engine rotates at a predetermined negative rotation speed or lower as a starter drive prohibition section.
Japanese Patent Publication No. 5442042 (Patent Document 2) discloses an engine starting apparatus
that estimates, in an engine stopping process, whether or not the engine rotates backward before the next predetermined crank position is reached, based on the rotation speed of the engine at a specific predetermined crank position, for example, at the top dead center (TDC) of a cylinder,
that specifies, after the estimation of backward rotation, the interval from the timing of starter drive prohibition to the timing of cancellation of starter drive prohibition as a prohibition range and specifies the interval out of the specified prohibition range as a permission range, and
that prohibits restarting of the engine in the foregoing specified prohibition range and restarts the engine in the foregoing specified permission range, when the restarting condition is established in an engine stopping process.
Moreover, Japanese Patent Application Laid-Open No. 2014-77399 (Patent Document 3) discloses an engine starting apparatus
that includes a first estimation means for estimating a forward engine rotation speed at a time when the rotation speed is decreasing and a second estimation means for estimating a backward engine rotation speed, and
that estimates the engine rotation speed in a rotation speed decreasing time, while performing switching between the first estimation means and the second estimation means in accordance with whether the engine output axle is rotating forward or backward.